The invention relates to the catalytic preparation of carboxylic acid esters via oxidative esterification.
The production of methyl methacrylate (MMA) from methacrolein (MAC), methanol, and oxygen is known. For example, U.S. Pat. No. 6,040,472 discloses this reaction using a palladium (Pd)—lead (Pb) crystalline structure, Pd3Pb1, on a silica support that has minor alumina and magnesia components. However, the Pd-Pb catalyst is capable of producing undesirably high amounts of methyl formate as a by-product. U.S. Pat. No. 4,518,796 discloses the use of a Pd—bismuth (Bi) catalyst. However, that catalyst did not give high MMA selectivity, which is desired for this reaction.
U.S. Pat. No. 5,892,102 discloses MAC oxidative esterification catalysts that include Pd—Bi—X intermetallics, where X can be a variety of elements, on a ZnO or CaCO3. These supports are undesirable from a mechanical stability, likely acid resistance, and long-term catalyst life standpoint.
It would be desirable to have a process for selectively producing MMA while producing very little methyl formate by-product, using a non-Pb catalyst (thereby avoiding the issues associated with Pb-containing waste streams) on a stable support.